In many applications, such as aerospace structures, it is desired to include a sensor system distributed along the structure to enable position and magnitude sensing of environmental effects in real time. This allows active damping of space structures and load monitoring and alleviation of aircraft structures to be built. The use of optical fibers as sensing elements is particularly desirable as they are light, tough and can be placed in or on a structure with a minimum of cost and degradation. These applications are discussed by E. Udd, in, "Fiber Optic Smart Structures", Fiber Optic Sensors: An Introduction for Engineers and Scientists edited by E. Udd, Wiley, (1991). Dakin et al in, "A Novel Distributed Optical Fibre Sensing System Enabling Location of Disturbances in a Sagnac Loop Interferometer", SPIE Vol. 838 Fiber Optic and Laser Sensors V (1987) pp. 325-326, describes a distributed fiber optic sensor based on the combination of a Sagnac interferometer and a Mach-Zehnder interferometer. The Sagnac interferometer has a position dependent response as described by E. Udd in, "Acoustic Sensor Based on the Sagnac Interferometer", Proceedings of SPIE, Vol. 425, 1983, pp. 90-95. Dakin et al showed that by combining the output of the Sagnac interferometer in response to a frequency dependent environmental effect along with the response of a Mach-Zehnder interferometer (see A. Dandridge, "The Mach-Zehnder and Michelson Interferometer" in Fiber Optic Sensor and Introduction for Engineers and Scientists, edited by E. Udd, Wiley (1991)) and normalizing the result, the position and location of a frequency dependent environmental effect along an optical fiber could be determined.
One of the major disadvantages of the Dakin et al approach is that the requirements on the light source for optimum performance of a Mach-Zehnder and Sagnac interferometer are diametrically opposed. Specifically, a high performance Mach-Zehnder interferometer needs a long coherence length light source that is often very susceptible to feed back while the Sagnac interferometer performs best with a low coherence length light source. While it is possible to reduce these problems by redesigning the Dakin et al distributed sensor using wavelength division multiplexing techniques and an isolator, it is also possible to design a distributed sensor based solely on the Sagnac interferometer as described by E. Udd in U.S. Pat. Nos. 4,898,468, 4,976,507 and 5,046,848.